Biology 11 E How Plants Colonized Land - Chapter 29
Name __________________
Overview: The Greening of Earth
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For the first 3 billion years of Earth’s history, the land was lifeless.
Thin coatings of cyanobacteria existed on land about 1.2 billion years ago.
About 500 million years ago, plants, fungi, and animals joined the cyanobacteria.
By about 385 million years ago, taller plants appeared, leading to the formation of the
first forests.
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More than 290,000 species of plants inhabit Earth today.
Most plants live in terrestrial environments, including mountaintops, deserts, and
aquatic habitats.
○ Even aquatic plants are referred to as land plants, to distinguish them from algae.
The presence of plants has enabled other organisms to survive on land.
○ Plants are the source of oxygen and the ultimate provider of food for land
animals.
○ Plant roots create habitats for other organisms by stabilizing soil.
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Vocabulary
adaptation, alternation of generations, colonial, dicots, enclosed seeds, flowers, fruit, leaves,
monocots, multicellular, pollen, roots, seeds, stems, unicellular, vascular tissue
Objectives:
Analyse how the increasing complexity of algae, mosses, and ferns represent an evolutionary
continuum of adaptation to a land environment
Analyse how the increasing complexity of gymnosperms and angiosperms contribute to
survival in a land environment
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Approximately how long ago did plants move onto land? ____________________ mya
This diagram shows how plants appear to be are related to various algae groups:
Which alga group are land plants most closely
related to? __Charophycean green algae_
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Many key traits of land plants also appear in
some protists, primarily algae.
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Plants are multicellular, eukaryotic,
photosynthetic autotrophs.
○ Red, brown, and some green algae also fit
this description.
Plants have cell walls made of cellulose.
○ So do green algae, dinoflagellates, and brown algae.
Plants have chloroplasts with chlorophyll a and b.
○ So do green algae, euglenids, and a few dinoflagellates.
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What is the evidence that charophycean green algae are the most closely related group of
algae to the land plants?
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The charophytes are the only algae that share the following four distinctive traits with
land plants, strongly suggesting that they are the closest relatives of plants.
1. The plasma membranes of both land plants and charophytes have distinctive rings
of cellulose-synthesizing complexes that synthesize the cellulose microfibrils of
the cell wall. These complexes contrast with the linear sets of cellulose-producing
proteins in noncharophyte algae.
2. Both charophytes and land plants have peroxisome enzymes to help minimize the
loss of organic products as a result of photorespiration. Peroxisomes of other
algae lack these enzymes.
3. The structure of flagellated sperm of charophytes and land plants with sperm is
very similar.
4. The formation of a phragmoplast is common only to land plants and the most
complex charophyte algae. A group of microtubules knows as a phragmoplast
forms between the daughter nuclei of a dividing cell. A cell plate develops in the
middle of the phragmoplast, giving rise to a new cross wall that separates the
daughter cells.
Plus: Comparisons of nuclear and chloroplast genes from a wide range of plants and
algae support the hypothesis that the charophytes are the closest living relatives of
land plants.
What traits link charophytes to land plants that allowed plants to live out of water?
Many charophyte algae inhabit shallow waters at the edges of ponds and lakes, where
they experience occasional drying. In such environments, natural selection favors
individuals that can survive periods when they are not submerged in water.
A layer of a durable polymer called sporopollenin prevents exposed charophyte zygotes
from drying out until they are in water again. This chemical adaptation may have
been the precursor to the tough sporopollenin walls that encase plant spores.
The accumulation of such traits by at least one population of ancestral charophytes
enabled their descendents—the first land plants—to live permanently above the
waterline.
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What evolutionary novelties of the first land plants opened up an expanse of terrestrial
habitat previously occupied only by films of bacteria.
○ The new frontier was spacious.
○ The bright sunlight was unfiltered by water and plankton.
○ The atmosphere had an abundance of CO2.
○ The soil was rich in some mineral nutrients.
○ Initially, there were relatively few herbivores or pathogens.
What were the challenges in the new environment?
Relative scarcity of water and
A lack of structural support against gravity.
A number of adaptations evolved in plants that allowed them to survive and reproduce on
land.
There are four key traits distinguish land plants from algae.
See figure 29.5 on page 602 and 603.
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3.
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Alternation of generations (along with multicellular, dependent embryos)
Sporangia that produce walled spores
Gametangia that produce gametes
Apical meristems
Some of these traits are not unique to plants; not every land plant exhibits all four
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traits.
This diagram illustrates the phylogeny of the major groups of plants. What is the
difference between non-vascular and vascular plants?
Vascular plants have veins, vascular tissue to conduct water and minerals upwards from
soil and monosaccharides from leaves to rest of plant.
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Bryophyta refer to the chart and posters on bryophytes
Examine the bryophytes in the lab as well as the bryophyte posters.
1. Provide examples
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2. Examine the moss specimens.
a. Sketch the male and female gametophytes separately. Add any labels that
you are able to.
b.
Sketch and label the sporophyte. You can examine the sporangium under
a dissecting microscope.
3. What is it that helps you to identify these plants as bryophytes? In other words
what defines the bryophytes?
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They are non-vascular plants.
Here is the life-cycle of a moss. Which generation is dominant in the moss life cycle?
__________Gametophyte___________________________
How are moss adapted to reproducing on land? Eggs are protected inside the
archegonium. Sperm are produced in the antheridium and swim through a film of water
toward the eggs, entering the archegonium in response to a chemical attractant. The
zygote develops inside the archegonium. Placental cells transport nutrients to the embryo.
How are moss still tied to a moist environment for their reproduction? ______________
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Sperm are produced in the antheridium and swim through a film of water toward the
eggs, entering the archegonium in response to a chemical attractant
What are some adaptations of moss for living on land? ___________________________
Rhizoids anchor gametophyte and do not play a major role in water and mineral
absorption. Spore dispersion from sporophyte when it dries out.
How does the moss plant body tie them
to a moist environment?
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No protection by a cuticle layer on the
leaf allows for easy moisture loss. No
vascular tissue to transport water and
nutrients. This occurs by diffusion
from cell to cell.
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p. 608
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Vascular plants (Tracheophytes)
To answer the question: What major adaptations do vascular plants
have for living and reproducing on land?
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Well developed vascular systems which allow them to grow
taller because
o 1. Water is conducted up the plant.
o 2. The vascular tissue contains xylem which has lignin in the cells that is
supportive.
o 3. Phloem transports nutrients throughout the plant.
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Life cycles with dominant sporophyte
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Well developed roots
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Well developed leaves.
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Spore bearing leaves called sporophylls which develop into cones and fruit.
Vascular plants such as ferns are seedless.
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Examine the ferns in the lab. Refer to the poster and charts in the lab
Are you looking at the sporophyte of gametophyte generation?
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Ferns are seedless vascular plants that are more well-adapted than moss to live in dry
environments. What are some adaptations shown by ferns for life on dry land?
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______vascular tissues
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Roots and
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Leaves
What are the distinguishing characteristics of this organism?
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What would you look for if you wanted to find a fern
gametophyte and where would you look?
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Examine a prepared slide of a fern gametophyte, Draw it and label male antheridia,
female archegonia and thallus.
Where are the spores produced?
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Using a blade scrape a small section of a sorus off a fern frond. Squash this on a
microscope slide under a coverslip. (If this is not possible use a prepared slide)
Draw and label what you see.
How are ferns still tied to a moist environment?
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Examine the specimen of horsetail. Would you classify it as a bryophyte or a
tracheophyte? ____________________
Provide at least one interesting fact about horsetail!
Horsetails grew up to 15 m high during the Carboniferous period
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Seedless vascular plants played an important role in the history of life on land.
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The ancestors of modern bryophytes and ferns, along with their seedless vascular relatives,
formed the first forests during the late Devonian and early Carboniferous periods.
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With the evolution of vascular tissue, roots, and leaves, these plants accelerated their rate of
photosynthesis and dramatically increased the removal of CO2 from the atmosphere.
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Scientists estimate that CO2 levels dropped by as much as a factor of 5 during the
Carboniferous period, causing global cooling and widespread glacier formation.
The first forests gave rise to modern-day coal.
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In the stagnant waters of the Carboniferous period, dead plants did not fully decay.
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The organic material turned to thick layers of peat. Marine sediments piled up on top, and
over millions of years, heat and pressure converted the peat to coal.
Humans still burn 6 billion tons of coal each year.
Do you see any irony in this?
Coal, formed from plants that contributed to global cooling, now contributes to global
warming by returning carbon to the atmosphere
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